1. Field of the Invention
The present invention relates to a photo-detecting device. In particular, the present invention relates to a photo-detecting device which provides fast photo-sensitive response, with a reduced photocurrent component (tail current) which has very slow response as compared to that of the majority of the photocurrent; and a method for producing the same.
2. Description of the Related Art
One class of photo-detecting devices having fast photo-sensitive response which are currently in wide use are so-called xe2x80x9cpin photodiodesxe2x80x9d. Pin photodiodes can be further classified depending on the type of material used as semiconductor material, i.e., xe2x80x9csilicon pin photodiodes xe2x80x9d, which are based on silicon, and xe2x80x9ccompound semiconductor pin photodiodesxe2x80x9d, which are based on compound semiconductor materials.
In general, a pin photodiode can be produced in the following manner.
First, a low concentration n-type semiconductor layer is allowed to grow its crystal on a high concentration n-type semiconductor substrate. Next, in predetermined regions which are to become island-like diffusion regions, a p-type impurity is diffused to some depth from the surface of the low concentration n-type semiconductor growth layer, thereby forming the island-like diffusion regions. Thereafter, a negative electrode is formed on the upper face of some of the islands of p-type diffusion regions, and a positive electrode is formed on the back face of the high concentration n-type semiconductor substrate. Thus, a pin photodiode is produced.
In the case of producing a compound semiconductor pin photodiode, e.g., InGaAs/InP, in particular, the low concentration n-type semiconductor growth layer may be formed in two layers. These two growth layers may include a light absorption layer which is adjacent to the semiconductor substrate, and a window layer formed on the light absorption layer, such that the window layer has a larger energy band gap than that of the light absorption layer. The size of the energy band gap can be adjusted by selecting the compound semiconductor material and appropriately changing the component ratios thereof. Next, a p-type impurity is diffused in the window layer to form island-like diffusion regions, whereby a compound semiconductor pin photodiode is produced. It should be noted that it is impossible to form such a window layer in a silicon pin photodiode structure because its energy band gap cannot be changed.
In a compound semiconductor pin photodiode having the above-described structure, regions of the light absorption layer which lie under the p-type diffusion regions function as photo-detecting portions. In the photo-detecting portions, a photocurrent is generated responsive to incident light which enters through the growth surface of the window layer.
Specifically, electron-hole pairs are generated through photoexcitation occurring in regions (photo-detecting portion) of the light absorption layer located under the p-type diffusion regions. The generated electron-hole pairs are dissociated by a potential barrier (electric field) at the p-n junction, so that the electrons migrate to the high concentration n-type semiconductor substrate and the holes migrate to the p-type diffusion regions. A photocurrent results from the migration of the electrons and the holes.
Compound semiconductor pin photodiodes which incorporate a window layer above a light absorption layer as mentioned above can provide an improved quantum efficiency because the window layer has a greater energy band gap than that of the light absorption layer so that the window layer becomes transparent with respect to the incident light, thereby preventing surface recombination of electron-hole pairs at the surface of the light absorption layer.
A photocurrent in a pin photodiode is primarily generated in the above-described manner. However, a photocurrent may also be generated in the case where light enters the window layer in regions other than the photo-detecting portions. Such a photocurrent may be generated due to the diffusion of holes, and has a response which is much slower than the photocurrent that is generated in the photo-detecting portions. This photocurrent having a very slow response is commonly referred to as a xe2x80x9ctail currentxe2x80x9d, which may present a significant problem in certain applications of the photo-detecting device. The mechanism which generates a tail current will be described below.
The light entering regions of the window layer other than those corresponding to the photo-detecting portions generate electron-hole pairs in the underlying light absorption layer. However, since no potential barrier (electric field) that is associated with a p-n junction exists in these regions, the generated electrons and holes migrate due to diffusion, rather than due to an electric field. That is, the generated electrons and holes diffuse in accordance with their respective density gradients so as to permeate the surrounding low concentration regions. Since the electrons are the majority carriers in the nxe2x88x92layer (i.e., light absorption layer), it is presumable that the electrons immediately create a photocurrent before even reaching the n-substrate. On the other hand, only those of the holes which have reached the p-type diffusion regions through diffusion create a photocurrent, whereas the other holes will recombine with the electrons over a long period of time. Since the holes have a long lifetime within the light absorption layer, some holes may reach the p-type diffusion layer after having diffused through the light absorption layer over a long period of time. A tail current is defined as a component of the photocurrent that is attributable to the diffusive migration of such holes.
As described above, the cause for a tail current is the electron-hole pairs generated in regions other than the photo-detecting portions. Therefore, in order to reduce the tail current, it has been proposed to construct a photo-detecting device in which regions other than photo-detecting portions are covered by a light-shielding film such as a thin metal film. This technique for reducing the tail current is generally employed in the field of silicon pin photodiodes.
However, the aforementioned technique is difficult to apply to compound semiconductor pin photodiodes due to the nature of the actual production processes. Specifically, the production of a compound semiconductor pin photodiode requires highly precise micro-processing techniques because a depletion layer for a compound semiconductor material is much narrower than a depletion layer for silicon, as described below in more detail.
In a photo-detecting device, regions other than photo-detecting portions are usually not entirely covered by a light-shielding film such as a thin metal film because such a light-shielding film (e.g., a thin metal film) would cause short-circuiting if they contact an annular electrode, wiring and/or a pad composed of a conductive material, which are formed on the surface of a photo-detecting device on which photo-detecting regions are formed. Rather, such a light-shielding film is provided so as to have a minimum interspace with each conductive element on the surface of the photo-detecting device. The interspaces, which cannot shield incident light, should be minimized in order to minimize the tail current. Specifically, such a light-shielding film is only required to be large enough so that its inner end (i.e., the end adjoining the interspace with a conductive element on the device surface) is in an overlapping relation with the outer periphery of an underlying depleted intrinsic semiconductor layer (i.e., a depletion layer), when viewed from above the light entering surface (i.e., the upper face of the substrate). In accordance with this configuration, even if light enters the depletion layer through the interspace, a very rapid response can be obtained because of the electric field applied to the depletion layer, so that no tail current is generated. Another advantage associated with the photo-detecting device structure in which the inner end of a light-shielding film is in an overlapping relation with the outer periphery of an underlying depletion layer is that no parasitic capacitance is additionally created.
In the case of a silicon pin photodiode, the depletion layer has a thickness of about 10 xcexcm or more. The depletion layer also expands not only along the vertical direction but also along the horizontal direction over a width of about 10 xcexcm or more in the vicinity of the photo-detecting regions. Therefore, for a silicon pin photodiode, the aforementioned interspace may be prescribed to be about 10 xcexcm in order to sufficiently restrain the tail current without allowing a parasitic capacitance to be additionally created.
On the other hand, in the case of a compound semiconductor pin photodiode the depletion layer has a thickness of only about 2 xcexcm, while expanding along the horizontal direction over a width of only about 2 xcexcm. Therefore, for a compound semiconductor pin photodiode, the aforementioned interspace must be prescribed to be about 2 xcexcm. Thus, the interspace should be prescribed to be much smaller for a compound semiconductor pin photodiode than for a silicon pin photodiode, which will require highly precise micro-processing techniques. In addition, the micro-processing techniques for compound semiconductors are generally not as advanced as those required for silicon. For these reasons, it is very difficult to produce a compound semiconductor pin photodiode such that the inner end of a light-shielding film is in an overlapping relation with the outer periphery of an underlying depletion layer.
Furthermore, when producing a light-shielding film adjacent to an end of a photo-detecting region, a smaller-than-prescribed interspace may be left between an annular electrode which is formed at the edge of the photo-detecting portion and the light-shielding film due to insufficient micro-processing accuracy. In such cases, a parasitic capacitance may be created between the light-shielding film and the annular electrode. In extreme cases, the light-shielding film may be short-circuited with the annular electrode.
As described above, the technique of covering regions other than photo-detecting portions with a light-shielding film cannot be easily applied to compound semiconductor pin photodiodes. However, Japanese Laid-Open Publication No. 3-276769 discloses one such attempt (hereinafter referred to as xe2x80x9cConventional Examplexe2x80x9d). FIG. 6A is a plan view of a compound semiconductor pin photodiode (photo-detecting device) 600 of Conventional Example. FIG. 6B is a cross-sectional view taken along line X-Y in FIG. 6A.
With reference to FIGS. 6A and 6B, the structure of the compound semiconductor pin photodiode 600 of Conventional Example, and a method for producing the same will be described.
First, an n-InP buffer layer 62, an n-InGaAs light absorption layer 63, and an n-InP window layer 64 are formed on an n-InP substrate 61 in this order. After forming a diffusion region 66 in a portion of the window layer 64 by diffusing a p-type impurity therein, a semi-insulative InP capping layer 67 is allowed to grow its crystal over the upper face of the diffusion region 66 and the window layer 64. A via hole is formed in the capping layer 67, in which a p-InGaAs conductive embedded layer 68 is allowed to grow its crystal. Then, a p-InGaAs wiring layer 69 and a p-InGaAs wire bonding portion 70 are formed on the capping layer 67 through p-InGaAs crystal growth. The semi-insulative InP capping layer 67 is employed for reducing the interlayer capacitance between the wiring layer 69 and the wire bonding section 70 and the n-InP window layer 64. Thereafter, an insulative film 71 (e.g., SiN) is formed on the wiring layer 69 and the exposed capping layer 67. Finally, a thin metal film (e.g., Ti/Au) is vapor-deposited and patterned into a pad 72 on the wire bonding section 70 and a light-shielding film 73 on the insulative film 71. A positive electrode 65 is formed on the back face of the substrate 61. This completes the compound semiconductor pin photodiode 600 of Conventional Example.
In accordance with the compound semiconductor pin photodiode 600 of Conventional Example, regions other than the photo-detecting portions 74 are basically covered by a thin metal film. As a result, the tail current of this device can be reduced to a certain extent.
In accordance with Conventional Example described above with reference to FIGS. 6A and 6B, the pad 72 and the light-shielding film 73 may be short-circuited when performing a wire bonding or flip-chip bonding step for the following reasons. In general, a pin photodiode is required to have a minimized device capacitance in order to operate at a high speed. Therefore, the pad area should be minimized as much as possible. Moreover, the interspace between the light-shielding film and the pad must be minimized in order to obtain a satisfactory light-shielding effect for reducing the tail current. However, designing a pin photodiode so as to have a small pad area and a minimized interspace between the pad and the light-shielding film invites a high possibility of short-circuiting between the pad and the light-shielding film because a deformed tip end portion of a wire, which is typically formed during the wire bonding step, may easily reach the light-shielding film. Even if a flip-chip bonding technique is employed to mount the pin photodiode, instead of wire bonding, there is a high possibility of short-circuiting between the pad and the light-shielding film due to misalignment between the photodiode chip and a bump on a wiring substrate.
A photo-detecting device according to the present invention includes: a semiconductor substrate; a multilayer structure formed on the semiconductor substrate; an island-like photo-detecting region formed in at least a portion of the multilayer structure, the island-like photo-detecting region having a central portion; and a light-shielding mask formed on the semiconductor substrate so as to shield from light a portion of the island-like photo-detecting region at least excluding the central portion, wherein the light-shielding mask comprises an upper metal film and a lower metal film, and the upper metal film and the lower metal film are at least partially isolated by an insulative film, the upper metal film and the lower metal film having different patterns.
In one embodiment of the invention, the upper metal film and the lower metal film each have an inner end portion located adjacent to the photo-detecting region; the inner end portion of the upper metal film is located more closely to the photo-detecting region, along a horizontal direction, than the inner end portion of the lower metal film; and the upper metal film is not provided in a further region which is located at a predetermined distance from the photo-detecting region, the further region being shielded from light by the lower metal film.
In another embodiment of the invention, the inner end portion of the upper metal film is in an overlapping relation with the photo-detecting region.
In still another embodiment of the invention, the upper metal film is electrically coupled with the lower metal film via an opening in the insulative film.
In still another embodiment of the invention, an outer end portion of the semiconductor substrate is shielded from light by the lower metal film, and the insulative film and the upper metal film are not formed at in the outer end portion of the semiconductor substrate.
Alternatively, the photo-detecting device according to the present invention includes: a semiconductor substrate; a light absorption layer and a window layer formed in this order on the semiconductor substrate; a diffusion region formed in an island-like shape in the window layer; a negative electrode formed on a portion of the diffusion region; an insulative film formed on a portion of the window layer at least excluding a central portion of the diffusion region; a pad formed on a region of the insulative film which is located at a predetermined distance from the diffusion region; wiring formed on the insulative film for electrically connecting the negative electrode with the pad; an upper metal film formed on the insulative film so as to surround the diffusion region without overlapping the wiring; and a lower metal film formed between the window layer and the insulative film, wherein the negative electrode, the pad, the wiring, and the upper metal film are formed from the same thin metal film.
In one embodiment of the invention, substantially the entire diffusion region above the semiconductor substrate at least excluding the central portion is shielded from light by at least one of the upper metal film and the lower metal film.
In another embodiment of the invention, the photo-detecting device further includes a positioning mark formed in a portion of the diffusion region above the semiconductor substrate at least excluding the central portion, the positioning mark being shielded from light by neither the upper metal film nor the lower metal film.
In still another embodiment of the invention, the photo-detecting device further includes a contact hole and a positive electrode, wherein the contact hole is located in a region where the window layer and the light absorption layer above the semiconductor substrate has been removed, and wherein the positive electrode comprises a portion of the lower electrode which is present in an exposed surface of the contact hole.
In still another embodiment of the invention, the photo-detecting device further includes a positive electrode formed on the semiconductor substrate and a back face metal formed on a back face of the semiconductor substrate.
In still another embodiment of the invention, the photo-detecting device further includes a side face metal film formed on at least one side face of the semiconductor substrate, the light absorption layer, and the window layer.
In another aspect of the invention, there is provided a method for producing a photo-detecting device including the steps of: growing a light absorption layer and a window layer on a semiconductor substrate in this order; forming an island-like diffusion region in the window layer by diffusing an impurity therein; depositing a lower metal film or a portion of the window layer excluding the island-like diffusion region; depositing an insulative film on the window layer and the lower metal film; forming an opening over the island-like diffusion region by partially etching away the insulative film; depositing and lifting off a thin metal film so as to simultaneously form a negative electrode, a pad, wiring, and an upper metal film.
In one embodiment of the invention, the lower metal film is a lamination film comprising Cr, Pt, and Au.
Thus, the invention described herein makes possible the advantages of (1) providing a photo-detecting device structure in which the chip surface, excluding a photo-detecting portion, yet including end portions of the chip, is shielded from light, while substantially eliminating the possibility of short-circuiting between a pad and a light-shielding film during a wire bonding or flip-chip bonding step, and whose device capacitance is prevented from increasing due to the incorporation of the light-shielding film; and (2) providing a method for easily producing the aforementioned photo-detecting device with a minimum increase in the number of production steps relative to methods for producing conventional compound semiconductor pin photodiodes.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.